1. Field of the Invention
The invention relates to an organic light-emitting diode display device, and more particularly, to an organic light-emitting diode display device and a driving method thereof. Although embodiments of the invention are suitable for a wide scope of applications, they are particularly suitable for reducing a residual image phenomenon and a motion image blurring phenomenon, and for compensating a voltage drop of a driving voltage in an organic light-emitting diode display device.
2. Discussion of the Related Art
Recently, flat display panels with reduced weight and size have been developed to eliminate disadvantages of a cathode ray tube display device. Such flat panel display devices include a liquid crystal display (hereinafter, referred to as “LCD”) device, a field emission display (hereinafter, referred to as “FED”) device, a plasma display panel (hereinafter, referred to as “PDP”) device, and an electro-luminescence (hereinafter, referred to as “EL”) display device.
In general, a PDP has been highlighted among flat panel display devices as advantageous to have light weight, a small size and a large dimension screen because its structure and manufacturing process are simple. However, a PDP has a low light-emission efficiency and requires large power consumption. Likewise, an active matrix LCD device employing a thin film transistor (hereinafter, referred to as “TFT”) as a switching device has experienced drawbacks in that it is difficult to make a large dimension screen because a semiconductor process is used, but has an expanded demand as it is mainly used for a display device of a notebook personal computer. On the other hand, an EL display device is largely classified into an inorganic EL display device and an organic light-emitting diode display device depending upon a material of a light-emitting layer. An EL display device also is advantageous in that it is self-luminous. When compared with the above-mentioned display devices, the EL device generally has a faster response speed, a higher light-emission efficiency, greater brightness and a wider viewing angle.
FIG. 1 is a schematic diagram illustrating a structure of an organic light-emitting diode display device according to the related art. In FIG. 1, the organic light-emitting diode device includes an anode electrode ANODE made of a transparent conductive material on a glass substrate, and a cathode electrode CATHODE made of an organic compound layer and a conductive metal. The organic light-emitting diode device also includes an organic compound layer. The organic compound layer comprises a hole injection layer HIL, a hole transport layer HTL, an emission layer EML, an electron transport layer ETL, and an electron injection layer EIL.
When a driving voltage is applied to the anode electrode ANODE and the cathode electrode CATHODE, a hole within the hole injection layer and an electron within the electron injection layer respectively move forward the emission layer EML to excite the emission layer EML. As a result, the emission layer EML emits visible rays and the visible rays generated from the emission layer EML display a picture or a motion picture.
The above-described organic light-emitting diode device has been applied to a passive matrix type display device or to an active matrix type display using a TFT as a switching element. The passive matrix type display device crosses the anode electrode ANODE with the cathode electrode CATHODE to select a light-emitting cell in accordance with a current applied to the anode and cathode electrodes ANODE and CATHODE. On the other hand, the active matrix type display device selectively turns-on an active element, such as a TFT, to select a light-emitting cell, and maintains light-emission in the light-emitting cell using a voltage maintained at a storage capacitor.
FIG. 2 is a circuit diagram illustrating a pixel of an active matrix type organic light-emitting diode display device according to the related art. Referring to FIG. 2, a pixel of an active matrix type organic light-emitting diode display device includes an organic light-emitting diode element OLED, a data line DL and a gate line GL that cross each other, a switch TFT T2, a driving TFT T1, and a storage capacitor Cst. The driving TFT T1 and the switch TFT T2 are made of a p-type MOS-FET.
The switch TFT T2 is turned-on in response to a gate low-level voltage (or a scanning voltage) from the gate line GL to form a current path between a source electrode and a drain electrode of the switch TFT T2, and maintains an off-state when a voltage of the gate line GL is less than a threshold voltage (hereinafter, referred to as “Vth”), that is, a gate high-level voltage. A data voltage from the data line DL is applied, via the source electrode and the drain electrode of the switch TFT T2, to a gate electrode and the storage capacitor Cst of the driving TFT T1 for an on-time period of the switch TFT T2. On the other hand, a current path between the source electrode and the drain electrode of the switch TFT T2 is opened for an off-time period of the switch TFT T2. As a result, the data voltage is not applied to the driving TFT T1 and the storage capacitor Cst.
In addition, the source electrode of the driving TFT T1 is connected to a driving voltage line VL and the storage capacitor Cst, and the drain electrode of the driving TFT T1 is connected to an anode electrode of the organic light-emitting diode element OLED. The gate electrode of the driving TFT T1 is connected to the drain electrode of the switch TFT T2. The driving TFT T1 adjusts a current amount between the source electrode and the drain electrode in accordance with the data voltage supplied to the gate electrode. As a result, the organic light-emitting diode element OLED emits brightness corresponding to the data voltage. Further, the storage capacitor Cst stores a difference voltage between the data voltage and a high-level driving voltage source VDD to maintain a constant voltage applied to the gate electrode of the driving TFT T1 for one frame period.
The organic light-emitting diode element OLED shown in FIG. 2 has the structure as shown in FIG. 1, and includes an anode electrode and a cathode electrode. The anode electrode of the organic light-emitting diode element OLED is connected to the drain electrode of the driving TFT T1, and the cathode electrode of the organic light-emitting diode element OLED is connected to a ground voltage source GND.
The brightness of a pixel as shown in FIG. 2 is in proportion to a current flowing into the organic light-emitting diode element OLED, and the current is adjusted by a voltage applied to the gate electrode of the driving TFT T1. In other words, a gate-source voltage |Vgs| between a gate electrode and a source element of the driving TFT T1 must be increased in order to improve brightness of a pixel. On the other hand, the gate-source voltage |Vgs| must be decreased in order to darken brightness of a pixel.
FIG. 3A is a graph illustrating a hysteresis characteristic of a thin film transistor according to the related art, FIG. 3B is an amplified graph of a portion of the graph shown in FIG. 3A, and FIG. 4 is a graph illustrating an example which an operating point of a thin film transistor is changed in accordance with a hysteresis characteristic. The driving TFT T1 (shown in FIG. 2) has a hysteresis characteristic. As shown in FIGS. 3A and 3B, the hysteresis characteristics are generated as a current between a drain electrode and a source electrode Ids changes in accordance with a change of a gate-source voltage |Vgs|. For example, if brightness of a pixel is changed from a white gray scale level to a middle gray scale level, then the gate-source voltage |Vgs| of the driving TFT T1 is changed from a high value to a low value. In this case, since a relatively high gate-source voltage |Vgs| is formerly applied to the driving TFT T1 at the white gray scale level, if the gate-source voltage |Vgs| corresponding to the middle gray scale level is applied to the driving TFT T1 at a state that a threshold voltage |Vth| of the driving TFT T1 is increased, then an operating point of the driving TFT T1 is changed as shown in “B” of FIG. 4.
On the other hand, if brightness of a pixel is changed from a black gray scale level to the middle gray scale level, then the gate-source voltage |Vgs| of the driving TFT T1 is changed from a low value to a high value. In this case, since a relative low gate-source voltage |Vgs| is formerly applied to the driving TFT T1 at the black gray scale level, if a gate-source voltage |Vgs| corresponding to the middle gray scale level is applied to the driving TFT T1 at a state that a threshold voltage |Vth| of the driving TFT T1 is decreased, then an operating point of the driving TFT T1 is changed as shown in “A” of FIG. 4. Accordingly, although the same gate-source voltage |Vgs| is applied to the driving TFT T1 to represent the same brightness of the middle gray scale level, different currents would flow to the organic light-emitting diode element OLED in accordance with a prior pixel brightness. Thus, a residual image is generated.
FIG. 5A is a diagram illustrating a test data according to the related art, FIG. 5B is a diagram illustrating an example of a residual image phenomenon after the test data shown in FIG. 5A is applied to the device shown in FIG. 2. FIG. 5A illustrates a test data displayed on a display screen when no residual image is generated. The test data is to display the white gray scale level and the black gray scale level that are arranged in a check pattern corresponding to pixels that are arranged in the matrix type organic light-emitting diode display device shown in FIG. 2. As shown in FIG. 5B, when a test data is applied to the organic light-emitting diode display device, a middle gray scale level data is instead displayed on the whole screen due to the hysteresis characteristic of the driving TFT.
Moreover, an active-type organic light-emitting diode display device according to the related art has a pixel configuration including TFTs and a storage capacitor as shown in FIG. 2 and is a hold type display. The hold type display device constantly maintains brightness of each pixel for each frame for one frame period as shown in FIG. 6. Thus, brightness of each pixel for one frame period is maintained, thereby burring an image of a motion picture and causing motion blurring. On the other hand, an impulse type display device, such as a cathode ray tube, emits light from the pixel for a time of one frame period, and does not emit light from the pixel for another frame period. As a result, a motion blurring phenomenon is almost not perceived by the observer.
In the active-type organic light-emitting diode display device, a current and brightness of the organic light-emitting diode element OLED is differentiated at a data having the same gray scale level in accordance with a screen position by a voltage drop. The voltage drop is generated by a driving voltage line VL supplying the high-level electric driving voltage source to each of the pixels. This phenomenon worsens as the driving voltage line VL becomes longer in a large size panel.